Exhaust gas purifying apparatus for internal combustion engine

ABSTRACT

An object of the present invention, in an internal combustion engine equipped with a moisture adsorbent and a NOx adsorbent in series in order from the upstream side of an exhaust passage, is to provide an exhaust gas purifying apparatus for the internal combustion engine which can successfully prevent an adsorption capability of NOx from being inhibited by moisture and is capable of maintaining the NOx adsorption capability of the NOx adsorbent adequately. A main exhaust passage  14  through which exhaust gas exhausted from an internal combustion engine  10  flows is provided. A bypass passage  20  bypassing the main exhaust passage  14  is provided. A moisture adsorbent  24  and a NOx adsorbent  28  are provided in the bypass passage  20  in series in order from a side closer to the upstream connecting portion  20   a . A switching valve  22  is controlled to prevent the exhaust gas from flowing into the NOx adsorbent  28  when it is judged that the desorption of the moisture from the moisture adsorbent  24  is started.

TECHNICAL FIELD

The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to an exhaust gas purifying apparatus including an adsorbent that is placed downstream of a catalyst in an exhaust passage for adsorbing unpurified components that cannot be purified by the catalyst.

BACKGROUND ART

A related technique for purifying ventilation gas exhausted from a road tunnel by adsorbing and removing NOx using a dry processing with a zeolitic adsorbent has been disclosed in the past, for example, by Patent Document 1. In the conventional technique, a silica-gel series dehumidifying agent (moisture adsorbent) is disposed to adsorb moisture contained in the ventilation gas.

[Patent Document 1] Japanese Laid-open Patent Application Publication No. Hei 1-155934

[Patent Document 2] Japanese Laid-open Patent Application Publication No. 2002-138820 DISCLOSURE OF INVENTION Problem to be Solved by the Invention

An exhaust passage of an internal combustion engine includes a catalyst for purifying exhaust gas. However, at the cold start when the temperature of the catalyst is low, the exhaust gas containing NOx may be exhausted outward until the catalyst is warmed and activated. The exhaust gas contains a large amount of moisture which is generated by the combustion of fuel. Consequently, as is the case with the conventional technique, it is conceivable to dispose a moisture adsorbent for adsorbing the moisture that harms a NOx adsorption capability of a NOx adsorbent at an upstream side of the exhaust gas flow in the exhaust passage of the internal combustion engine, and to dispose the NOx adsorbent downstream of the moisture adsorbent.

As described above, in the configuration equipped with the moisture adsorbent and the NOx adsorbent in the exhaust passage, if an adsorbing amount of the moisture in the moisture adsorbent has reached to a saturation point, or if an amount of moisture that exceeds the moisture adsorbing capability of the moisture adsorbent flows into the moisture adsorbent, the moisture that cannot be adsorbed by the moisture adsorbent flows into the NOx adsorbent disposed downstream thereof. When the moisture flows into the NOx adsorbent, the NOx adsorption by the NOx adsorbent is largely obstructed. Consequently, the desorption of NOx that is adsorbed in the NOx adsorbent is likely to occur.

The present invention has been made to solve the above problem. It is an object of the present invention, in an internal combustion engine equipped with a moisture adsorbent and a NOx adsorbent in series in order from the upstream side of an exhaust passage, to provide an exhaust gas purifying apparatus for the internal combustion engine which can successfully prevent an adsorption capability of NOx from being harmed by moisture and is capable of maintaining the NOx adsorption capability of the NOx adsorbent adequately.

Means for Solving the Problem

A first aspect of the present invention is an exhaust gas purifying apparatus for an internal combustion engine, the apparatus comprising:

a main exhaust passage through which exhaust gas exhausted from the internal combustion engine flows;

a bypass passage branching off from the main exhaust passage at an upstream connecting portion connected to the main exhaust passage while merging again with the main exhaust passage at a downstream connecting portion provided downstream of the upstream connecting portion;

flow path switching means that is capable of switching a flow target into which the exhaust gas flows between the main exhaust passage and the bypass passage;

a moisture adsorbent that is disposed in the bypass passage and has a function of adsorbing moisture;

a NOx adsorbent that is disposed in the bypass passage at a downstream side of the moisture adsorbent as for an exhaust gas flow in a state where the moisture adsorbent adsorbs the moisture and has a function of adsorbing NOx;

moisture desorption judgment means for judging whether desorption of the moisture from the moisture adsorbent is started; and

flow path control means for controlling the flow path switching means to prevent the exhaust gas from flowing into the NOx adsorbent when it is judged that the desorption of the moisture from the moisture adsorbent is started.

A second aspect of the present invention is the exhaust gas purifying apparatus for the internal combustion engine according to the first aspect of the present invention,

wherein the moisture desorption judgment means judges whether the desorption of the moisture is started on the basis of a temperature of the moisture adsorbent.

ADVANTAGES OF THE INVENTION

According to the first aspect of the present invention, an adsorbing operation of NOx by the NOx adsorbent is terminated when it is judged that the desorption of the moisture from the moisture adsorbent is started so that the desorption of NOx due to the moisture is suppressed.

According to the second aspect of the present invention, it is possible to certainly terminate the adsorption of NOx before NOx starts to desorb from the NOx adsorbent by estimating inflow of the moisture to the NOx adsorbent on the basis of not the temperature of the NOx adsorbent but the temperature of the moisture adsorbent placed upstream thereof so that the judgment concerning the switching of the switching valve is performed more rapidly.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram explaining a configuration of an internal combustion engine system having an exhaust gas purifying apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram explaining an operation of the system according to the first embodiment of the present invention.

FIG. 3 is a timing chart for showing how a control to terminate the adsorbing operation of NOx is performed according to the first embodiment of the present invention.

FIG. 4 is a flowchart illustrating a routine that is executed in the first embodiment of the present invention.

FIG. 5 is a timing chart for showing how a control to terminate the adsorbing operation of NOx is performed according to the second embodiment of the present invention.

FIG. 6 is a flowchart illustrating a routine that is executed in the second embodiment of the present invention.

DESCRIPTION OF SYMBOLS

-   -   10 internal combustion engine     -   12 intake passage     -   14 main exhaust passage     -   16 front stage catalyst     -   18 rear stage catalyst     -   20 bypass passage     -   20 a upstream connecting portion     -   20 b downstream connecting portion     -   22 switching valve     -   24 moisture adsorbent     -   28 NOx adsorbent     -   30 return passage     -   32 purge control valve     -   40 Electronic Control Unit (ECU)

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment Description of System Configuration

FIG. 1 is a diagram explaining a configuration of an internal combustion engine system having an exhaust gas purifying apparatus according to a first embodiment of the present invention. The internal combustion engine 10 shown in FIG. 1 includes an intake passage 12 for taking air into a cylinder, and an exhaust passage through which exhaust gas exhausted from the cylinder flows.

The exhaust passage of the present embodiment includes a main exhaust passage 14 for exhausting the exhaust gas from the cylinder, and a bypass passage 20 described later. In the main exhaust passage 14, a front stage catalyst (SC) 16 and rear stage catalyst (UF) 18 that can purify the exhaust gas is placed in series in order from the upstream side.

The system of the present embodiment has the bypass passage 20 as a passage bypassing the main exhaust passage 14. The bypass passage 20 is configured to branch off from the main exhaust passage 14 at an upstream connecting portion 20 a placed downstream of the rear stage catalyst 18, and merge again with the main exhaust passage 14 at a downstream connecting portion 20 b placed downstream of the upstream connecting portion 20 a. The upstream connecting portion 20 a is provided with a switching valve 22 for switching a flow target into which the exhaust gas flows between the main exhaust passage 14 and the bypass passage 20. The opening and closing of the switching valve 22 is controlled by controlling an engine intake vacuum pressure, which acts on a negative pressure diaphragm 22 a, with an electromagnetic valve (not shown). Incidentally, at the normal operation of the internal combustion engine 10, by controlling the switching valve 22 to block the bypass passage 20, the exhaust gas passes through the main exhaust passage 14 without passing through the bypass passage 20 and is then released into the atmosphere.

The middle of the bypass passage 20 is provided with two adsorbents. More specifically, a moisture adsorbent 24 having a function of adsorbing moisture contained in the exhaust gas is installed at an upstream side of the main exhaust passage 14, that is, at a side closer to the upstream connecting portion 20 a. Zeolitic materials, for example, can be used as such moisture adsorbent 24. Into the moisture adsorbent 24, a temperature sensor 26 is integrated to detect a temperature of the moisture adsorbent 24.

In addition, in the bypass passage 20, a NOx adsorbent 28 having a function of adsorbing NOx, which is an unpurified component contained in the exhaust gas, is installed at a downstream side of the moisture adsorbent 24, that is, at a side farther to the upstream connecting portion 20 a. Zeolitic materials which support iron Fe, for example, can be used as such NOx adsorbent 28.

A part between the upstream connecting portion 20 a and moisture adsorbent 24 in the bypass passage 20 communicates with a return passage 30. A purge control valve 32 is provided in the middle of the return passage 30. The remaining end of the return passage 30 communicates with the intake passage 12. Incidentally, a connection point of the return passage 30 is not limited to the intake passage 12. A passage upstream of the rear stage catalyst 18, for instance, an upstream portion of the front stage catalyst 16 in the main exhaust passage 14, may be used as the connection point.

The system of the present embodiment includes an electronic control unit (ECU) 40. A water temperature sensor 42 for detecting a temperature of engine cooling water is connected to the ECU 40, as well as various sensors for controlling the internal combustion engine 10 and the above temperature sensor 26. In addition, various actuators such as the switching valve 22 and the purge control valve 32 mentioned above are connected to the ECU 40.

[Operation of First Embodiment]

FIG. 2 is a diagram explaining an operation of the system according to the first embodiment of the present invention.

(Adsorbing Operation)

First, with reference to FIG. 2(A), an operation for causing the above adsorbents 24, 28 to adsorb NOx and the moisture contained in the exhaust gas exhausted from the cylinder at the cold start of the internal combustion engine 10 will be described.

As shown in FIG. 2(A), the adsorbing operation is started in a state where the switching valve 22 blocks the main exhaust passage 14 at the cold start of the internal combustion engine 10. In addition, the purge control valve 32 is controlled to become a closed condition under the adsorbing operation.

In the state mentioned above, all of the exhaust gas exhausted from the internal combustion engine 10 is supplied from the main exhaust passage 14 into the bypass passage 20. The exhaust gas supplied into the bypass passage 20 passes through the moisture adsorbent 24 and the NOx adsorbent 28 in sequence and is then returned to the main exhaust passage 14 and is then released into the atmosphere.

If the amount of exhaust gas that exceeds the adsorption capability of the NOx adsorbent 28 is supplied to the NOx adsorbent 24, NOx adsorbed once by the NOx adsorbent 24 is desorbed and exhaust emission is caused to deteriorate. Therefore, the adsorbing operation needs to be terminated at an appropriate timing before the desorption of NOx from the NOx adsorbent 28 starts. A setting of the timing to terminate such adsorbing operation of NOx is a feature portion of the present embodiment and will be described later with reference to FIG. 3. In addition, if such a timing to terminate the adsorbing operation has come, the switching valve 22 is controlled to become a state where the bypass passage 20 is blocked.

According to the adsorbing operation described above, the moisture contained in the exhaust gas is adsorbed by the moisture adsorbent 24 and removed. Furthermore, NOx contained in the exhaust gas is adsorbed by the NOx adsorbent 28 and removed. This can prevent NOx from being released into the atmosphere at the cold start when the front stage catalyst 16 has not yet been activated.

(Purging Operation)

FIG. 2(B) is a diagram explaining a purging operation according to the present embodiment. As shown in FIG. 2(B), the purging operation according to the present embodiment is performed by using a method which flows NOx adsorbed by the NOx adsorbent 28 back into the intake passage via the return passage 30.

As shown in FIG. 2(B), the purging operation is started by opening the purge control valve 32, at a time when a predetermined purge start timing has come, for example, the front stage catalyst 16 has been activated, in a state where the switching valve 22 is controlled to block the bypass passage 20. According to such purging operation, part of the exhaust gas exhausted from the cylinder is supplied from the main exhaust passage 14 into the bypass passage 20 via the downstream connecting portion 20 b, by using a vacuum being generated in the intake passage 12 of the internal combustion engine 10.

As a result, the exhaust gas relatively heated after the start is supplied to the adsorbent 28 or the like. Accordingly, NOx is desorbed from the NOx adsorbent 28 and purged to the intake passage 12 via the return passage 30. NOx returned to the intake passage 12 is burned again and then purified by the active catalyst 16 or the like.

[Feature Portions of the First Embodiment]

In the present embodiment, the moisture adsorbent 24 is disposed upstream of the NOx adsorbent 28 at the adsorbing operation as shown in FIG. 2. By such a layout, the moisture is preliminarily removed by the moisture adsorbent 24 at the upstream of the NOx adsorbent 28. This can allow a dried exhaust gas to supply to the NOx adsorbent 28 and can highly maintain the NOx adsorbing performance of the NOx adsorbent 28.

At the adsorbing operation, however, if an adsorbing amount of moisture in the moisture adsorbent 24 has reached to a saturation point, or if an amount of moisture that exceeds the moisture adsorbing capability of the moisture adsorbent 24 flows into the moisture adsorbent 24, the moisture that cannot be adsorbed by the moisture adsorbent 24 flows into the NOx adsorbent 28 disposed downstream thereof. When the moisture flows into the NOx adsorbent 28, the NOx adsorption capability of the NOx adsorbent 28 is largely harmed. Consequently, the desorption of NOx that is adsorbed by the NOx adsorbent 28 is likely to occur.

The present embodiment controls the switching valve to block the inlet of the bypass passage 20 and terminates the adsorbing operation of NOx, at a time when it is judged that the desorption of the moisture from the moisture adsorbent 24 which is installed upstream of the NOx adsorbent is started. More specifically, when performing the adsorbing operation, the present embodiment judges a time when the desorption of the moisture from the moisture adsorbent 24 is started by judging whether a temperature of the moisture adsorbent 24 detected by the temperature sensor 26 has become equal to or higher than a predetermined value.

FIG. 3 is a timing chart for showing how a control to terminate the adsorbing operation of NOx is performed according to the first embodiment of the present invention. As shown in FIG. 3, when the moisture adsorbent 24 adsorbs the moisture by introducing the exhaust gas including the moisture, the temperature of the moisture adsorbent 24 increases due to the adsorption heat. However, when the moisture adsorbent 24 adsorbs the moisture larger than a certain amount, it cannot adsorb any more moisture. Therefore, the moisture flows into the NOx adsorbent 28.

A predetermined value T1 shown in FIG. 3 concerning the moisture adsorbent 24 is a value used to judge a time when a certain amount of moisture which coincides with such an amount that the desorption of the moisture from the moisture adsorbent 24 is started is adsorbed by the moisture adsorbent 24. The present embodiment, at a time when the temperature of the moisture adsorbent 24 has reached the predetermined value T1, changes positions of the switching valve 22 so that a state where the bypass passage 20 is opened (a state of “with adsorption” shown in FIG. 3) varies to a state where the bypass passage 20 is closed (a state of “without adsorption” shown in FIG. 3).

FIG. 4 is a flowchart illustrating a routine that the ECU 40 executes in order to implement the adsorbing operation and purging operation according to the present embodiment. The routine shown in FIG. 4 is started immediately after the internal combustion engine 10 is started.

In the routine shown in FIG. 4, step 100 is first performed to judge whether the temperature of the engine cooling water is equal to or lower than a predefined temperature. As a result, if the judgment result indicates that the temperature of the engine cooling water is higher than the predefined temperature, that is, if it can be judged that the engine warm-up has been completed, the current processing cycle immediately terminates.

If, on the other hand, the judgment result obtained in step 100 indicates the temperature of the engine cooling water is equal to or lower than the predefined temperature, that is, if it can be judged that the engine is in a cold start condition, step 102 is performed to open the switching valve 22 and close the purge control valve 32. The switching valve 22 at the normal operation blocks the inlet of the bypass passage 20. However, if it can be judged that the engine is in the cold start condition, step 102 is performed so that the main exhaust passage 14 may communicate with the bypass passage 20. Thus, the adsorbing operation is started.

Next, step 104 is performed to judge whether the temperature of the moisture adsorbent 24 has reached a predetermined value T1. Incidentally, the temperature of the moisture adsorbent 24 is directly measured by the temperature sensor 26 here. But, an alternative method that estimate the temperature of the moisture adsorbent 24 based on the anterior and posterior exhaust gas temperatures of the moisture adsorbent 24 may be used.

If the judgment result obtained in step 104 indicates that the temperature of the moisture adsorbent 24 has reached the predetermined value T1, that is, if it can be judged that the moisture adsorbent 24 is in a state where the desorption of the moisture is started, step 106 is performed to close the switching valve 22 in order to block the bypass passage 20. Thus, the adsorbing operation of NOx is ended.

Next, step 108 is performed to judge whether the start timing of the purging operation has come. More specifically, step 108 performs a judgment based on whether the catalyst 16 or the like is in an active state, a judgment based on whether the temperature of the exhaust gas supplied to the bypass passage 20 is within a temperature range suitable for performing the purging operation, and a judgment based on whether the internal combustion engine 10 is in a stable operating state that can perform the purging operation without an adverse effect.

If the judgment result obtained in step 108 indicates that the start timing of the purging operation has come, step 110 is performed to open the purge control valve 32. Next, the purge amount judgment is performed in step 112. More specifically, it is judged whether the current purge amount has reached a predefined amount. The current purge amount can be estimated on the basis of the relation between the temperature of the purge gas, and the elapsed time after the purge control valve 32 is opened in step 110 described above.

If the judgment result obtained in step 112 indicates that the purge amount has reached the predefined amount, step 114 is performed to close the purge control valve 32. Thus, the purging operation is ended.

According to the routine that has been described above with reference to FIG. 4, the switching valve 22 is operated when the temperature of the moisture adsorbent 24 has reached the predetermined value T1, (that is, on the basis of the estimation results of the adsorption capability of the NOx adsorbent 28 using the temperature of the moisture adsorbent 24). This makes it possible to terminate the adsorbing operation of NOx when the desorption of the moisture from the moisture adsorbent 24 starts. According to such a control, it is possible to terminate the adsorption operation of NOx, immediately before the moisture flows into the NOx adsorbent 28, that is, before NOx adsorbed by the NOx adsorbent 28 is desorbed due to the moisture. Therefore, the NOX adsorption capability of the NOx adsorbent 28 can be utilized at a maximum.

In addition, the routine described above can certainly terminate the adsorption of NOx before NOx starts to desorb from the NOx adsorbent 28 by estimating the inflow of the moisture to the NOx adsorbent 28 based on not the temperature of the NOx adsorbent 28 but the temperature of the moisture adsorbent 24 placed upstream thereof so that the judgment concerning the switching of the switching valve 22 is performed more rapidly.

Incidentally, in the first embodiment, which has been described above, the switching valve 22, the return passage 30, and the purge control valve 32 correspond to the “flow path switching means” according to the first aspect of the present invention. In addition, the “moisture desorption judgment means” according to the first aspect of the present invention is implemented when the ECU 40 performs step 104; and the “flow path control means” according to the first aspect of the present invention is implemented when the ECU 40 performs step 106.

Second Embodiment

Next, a second embodiment of the present invention will now be described with reference to FIGS. 5 and 6.

The system according to the present embodiment is implemented by adopting the hardware configuration shown in FIG. 1 and by allowing the ECU 40 to execute a routine shown in FIG. 6 described below instead of the routine shown in FIG. 4.

[Feature Portions of the Second Embodiment]

FIG. 5 is a timing chart for showing how a control to terminate the adsorbing operation of NOx is performed according to the second embodiment of the present invention.

The first embodiment described above judges the start timing of the moisture desorption from the moisture adsorbent 24 by judging whether the temperature of the moisture adsorbent 24 at the adsorbing operation has reached the predetermined value T1. In contrast, the present embodiment judges the start timing of the moisture desorption from the moisture adsorbent 24 by judging whether a temperature-elevation degree of the moisture adsorbent 24 at the adsorbing operation has become smaller than a predetermined value X1.

As described above, when the moisture is adsorbed by the moisture adsorbent 24, the temperature of the moisture adsorbent 24 increases due to the adsorption heat. On the other hand, when the adsorbed moisture desorbs from the moisture adsorbent 24, the temperature of the moisture adsorbent decreases due to the desorption heat. Consequently, if the desorption amount is larger than the adsorption amount as the inflow amount of moisture to the moisture adsorbent 24 increases, the temperature-elevation degree of the moisture adsorbent 24 decreases.

The predetermined value X1 shown in FIG. 5 concerning the temperature-elevation degree of the moisture adsorbent 24 is a value used to judge a time when a certain amount of moisture which coincides with such an amount that the desorption of the moisture from the moisture adsorbent 24 is started is adsorbed by the moisture adsorbent 24.

The present embodiment, at a time when the temperature-elevation degree of the moisture adsorbent 24 has become smaller than the predetermined value X1, changes positions of the switching valve 22 so that a state where the bypass passage 20 is opened (a state of “with adsorption” shown in FIG. 5) varies to a state where the bypass passage 20 is closed (a state of “without adsorption” shown in FIG. 5).

FIG. 6 is a flowchart illustrating a routine that the ECU 40 executes in order to implement the adsorbing operation and purging operation according to the present embodiment. The routine shown in FIG. 6 is started immediately after the internal combustion engine 10 is started. In addition, as regards the steps in FIG. 6 that are the same as those in FIG. 4 according to the first embodiment, their description is omitted or abridged with the same reference numerals assigned.

As shown in FIG. 6, the adsorbing operation is started by performing step 102 to open the switching valve 22 after the cold start of the internal combustion engine 10. Then, step 200 is performed to judge whether the temperature-elevation degree of the moisture adsorbent 24 has become smaller than the predetermined value X1.

As a result, if the temperature-elevation degree of the moisture adsorbent 24 has become smaller than the predetermined value X1, that is, if it can be judged that the moisture adsorbent 24 is in a state where the desorption of the moisture is started, step 106 is performed to close the switching valve 22 in order to block the bypass passage 20. Thus, the adsorbing operation of NOx is ended.

After that, steps 108 to 114 of the routine shown in FIG. 6 are sequentially performed. Because these processes are the same as those in the routine shown in FIG. 4, their detailed description is omitted here.

According to the routine that has been described above with reference to FIG. 6, the switching valve 22 is operated when the temperature-elevation degree of the moisture adsorbent 24 has become smaller than the predetermined value X1. This makes it possible to terminate the adsorbing operation of NOx when the desorption of the moisture from the moisture adsorbent 24 starts. According to such a control, it is possible to terminate the adsorption operation of NOx, immediately before the moisture flows into the NOx adsorbent 28, that is, before NOx adsorbed by the NOx adsorbent 28 is desorbed due to the moisture. Therefore, the NOX adsorption capability of the NOx adsorbent 28 can be utilized at a maximum. 

1. An exhaust gas purifying apparatus for an internal combustion engine, the apparatus comprising: a main exhaust passage through which exhaust gas exhausted from the internal combustion engine flows; a bypass passage branching off from the main exhaust passage at an upstream connecting portion connected to the main exhaust passage while merging again with the main exhaust passage at a downstream connecting portion provided downstream of the upstream connecting portion; flow path switching means that is capable of switching a flow target into which the exhaust gas flows between the main exhaust passage and the bypass passage; a moisture adsorbent that is disposed in the bypass passage and has a function of adsorbing moisture; a NOx adsorbent that is disposed in the bypass passage at a downstream side of the moisture adsorbent as for an exhaust gas flow in a state where the moisture adsorbent adsorbs the moisture and has a function of adsorbing NOx; moisture desorption judgment means for judging whether desorption of the moisture from the moisture adsorbent is started; and flow path control means for controlling the flow path switching means to prevent the exhaust gas from flowing into the NOx adsorbent when it is judged that the desorption of the moisture from the moisture adsorbent is started.
 2. The exhaust gas purifying apparatus for the internal combustion engine according to claim 1, wherein the moisture desorption judgment means judges whether the desorption of the moisture is started on the basis of a temperature of the moisture adsorbent.
 3. An exhaust gas purifying apparatus for an internal combustion engine, comprising: a main exhaust passage through which exhaust gas exhausted from the internal combustion engine flows; a bypass passage branching off from the main exhaust passage at an upstream connecting portion connected to the main exhaust passage while merging again with the main exhaust passage at a downstream connecting portion provided downstream of the upstream connecting portion; a flow path switching device that is capable of switching a flow target into which the exhaust gas flows between the main exhaust passage and the bypass passage; a moisture adsorbent that is disposed in the bypass passage and has a function of adsorbing moisture; a NOx adsorbent that is disposed in the bypass passage at a downstream side of the moisture adsorbent as for an exhaust gas flow in a state where the moisture adsorbent adsorbs the moisture and has a function of adsorbing NOx; a moisture desorption judgment device for judging whether desorption of the moisture from the moisture adsorbent is started; and a flow path control device for controlling the flow path switching device to prevent the exhaust gas from flowing into the NOx adsorbent when it is judged that the desorption of the moisture from the moisture adsorbent is started. 